Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, FIG. 1 depicts a chart 10 of the specific energy and energy density for a number of different battery chemistries. As evident from chart 10, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. The high specific energy results from the low atomic weight of lithium (6.94).
In addition to high specific energy and energy density, Li-ion batteries charge faster than other battery types. Consequently, Li-ion based technologies are successfully replacing the well-established Nickel-Cadmium (Ni—Cd) and Nickel Metal Hydride (NiMH) technologies.
A typical prior art Li-ion cell 20 is depicted in FIG. 2. The cell 20 includes a cathode 22, an anode 24, and, in non-solid constructions, a separator 26. The cathode 22 is usually a lithium metal oxide like LiCoO2 wherein Li atoms 28 chemically form a complex compound. The anode 24 is usually graphite or some other material wherein lithium atoms 28 are physically accommodated within the vacancies in the anode material (intercalate) but do not chemically bond. The separator 26 is provided to prevent physical contact between the cathode 22 and the anode 24 while allowing for lithium ion transport. In the cell 20, lithium transport is effected by an electrolyte 30 within the cathode 22, the anode 24, and the separator 26. A typical electrolyte is LiFePO4. Thus, while FIG. 2 depicts the lithium atoms 28 within the electrolyte 30, the lithium atoms 28 in the electrolyte 30 are in the form of a lithium ion.
The cell 20 further includes a metal current collector 32 for the cathode 22 and a metal current collector 34 for the anode 24. The metal current collector 32 and the metal current collector 34 are used to connect the cell 20 to an external circuit 36 which in FIG. 2 is a charging circuit.
During charging of the cell 20 the positive potential of the charging circuit 36 connected to the cathode 22 forces the Li-ions from the LiCoO2 complex to migrate via the electrolyte 30 in the direction of the arrows 38 and intercalate (insert into interstitial or other vacancies) in the material used to form the anode 24. This ionic movement from the cathode 22 to the anode 24 via the electrolyte 30 is balanced by an electronic movement from the cathode 22 to the anode 24 via the external circuit 36 as indicated by the arrow 40.
Upon connection of an external circuit 42 in the form of an electrical load as depicted in FIG. 3, the above described process reverses. Accordingly, the lithium atoms 28 diffuse back from the anode 24 to the cathode 22 in ionic form via the electrolyte 30 in the direction of the arrows 44. The ionic flow of lithium 28 is balanced by an electronic flow through the external circuit 42 from anode 24 to the cathode 22 as indicated by the arrow 46.
As noted above, the lithium atoms 28 intercalate into the anode material. The physical insertion of the lithium atoms 28 into vacancies within the material results in a volume expansion of the anode material. Consequently, in addition to the anode material being selected based upon being electrochemically stable as a part of an electrochemical reaction, the anode material should also be physically stable and tolerant to the resulting volume expansion from intercalation of the lithium atoms. A poorly selected anode material will exhibit fracture resulting in premature failure of the battery or cell.
One material which is generally considered to be acceptable as an anode material is graphite. Graphite is structurally made of several parallel sheets of graphene held together loosely by weak Van der Waals force. Li atoms easily intercalate between these layers of graphene, resulting in only 10% volumetric expansion with no “deconstruction” or failure of the original Graphite material.
Another consideration in selection of an anode material is the formation of a solid electrolyte interphase (SEI) on the anode material. Since most electrolyte materials (like LiFePO4) are unstable in the presence of the chemical potential at the anode, the electrolyte materials decompose at the anode surface into lithium containing organic and inorganic compounds. This layer has been observed even on graphite anode materials.
The SEI is typically fully formed after one or two cycles of lithiation from the electrolyte onto the anode. The SEI is electrically insulating in nature while it maintains a good ionic conductivity. Formation of the SEI also prevents further decomposition of the electrolyte at the anode. If the SEI layer did not form, the electrolyte would totally decompose resulting in failure of the battery. Thus, the formation of the SEI is crucial for the feasibility of Li-ion batteries. Consequently, the proper chemistry and anode materials are necessary to generate an SEI which is not overly thick, resulting in increased resistance.
Silicon is another material which has been analyzed for use in a lithium intercalation battery. Silicon has the largest specific capacity known of any anode material as exhibited by the chart 50 of FIG. 4 and the chart 52 of FIG. 5. Silicon accommodates a massive amount of Li via intercalation phases (several phases exist like Li12Si7, Li7Si3, Li15Si4 etc.). Thus it is possible to store an enormous charge density of about 3272 mAh/g.
Silicon also has a very high gravimetric energy density, theoretically able to intercalate about 4.4 Li atoms for every Si atom. (Compared to 1 Li atom for every 6 carbon atoms in graphite). The high gravimetric energy density comes as a cost, however, as silicon exhibits a 280% volume expansion. The extreme volume expansion leads to sudden material fracture and battery failure even after a mere 50 cycles of charging and discharging.
The extreme volume expansion of silicon also results in destruction of the SEI which cracks as the base silicon material expands. Subsequently, a new SEI layer is formed on the freshly exposed silicon, and then destroyed during the next lithiation cycle. Consequently, the electrolyte is constantly being depleted.
One attempt to mitigate the effects of the extreme expansion of silicon has been reported by Szczech et al., “Nanostructured Silicon for High Capacity Lithium Battery Anodes,” The Royal Society of Chemistry, Vol. 4, pp. 56-72, 2011 and by Takur et al., “Inexpensive Method for Producing Macroporous Silicon Particulates with Pyrolyzed Polyacrylonitrile For Lithium Ion Batteries,” Nature—Scientific Reports, Vol. 2, No. 795. pp. 1-7, 2012. In these investigations, the silicon surface was nanostructured to convert the silicon surface into a sponge-like material that could absorb more Li ions and be able to relax the strain resulting from the volume expansion. Other nano-structuring includes porosification of silicon (nanoporous, mesoporous, macroporous silicon), silicon nanowires, silicon nanoparticles etc. While nano-structuring addresses the issue of expansion, it does not address the issue of SEI destruction.
Other investigations have been made into providing an artificial SEI which is less susceptible to fracturing. For example, Nguyen et al., “Alumina-coated Silicon-based Nanowire Arrays for High Quality Li-Ion Battery Anodes,” Journal of Material Chemistry, Vol. 22, pp. 24618-24626, 2012 report upon the use of an ultra-thin layer of atomic layer deposited (ALD) alumina (Al2O3) coated onto a silicon nanowire to act as an “artificially introduced” thin insulating SEI layer. While the ALD alumina layer improved performance of the silicon nanowire, the ALD alumina layer on the nanowires as reported by Nguyen et al. still exhibited cracking.
While there are significant challenges in using silicon as an anode material, the potential of incorporating silicon makes further research appealing. The capacity and size of a battery incorporating silicon would enable chip-scale micro-batteries (like integrated circuits, MEMS devices etc.,) that could power devices like MEMS sensors, CMOS memories, Smart Cards, Smart Dust, Drug Delivery Systems, Medical Implantable Devices etc.
What is needed therefore is a lithium intercalation type battery which incorporates silicon as an anode material. A lithium intercalation type battery with silicon as an anode material which exhibits a stable SEI is also desired.